Method of casting aerated concrete products

ABSTRACT

IN A METHOD OF CASTING AERATED CONCRETE PRODUCTS WHEREIN A FIRST AERATED CONCRETE MASS LAYER IS CAST AND PERMITTED TO SET OR HARDEN INTO A SELF-SUPPORTING CONSISTENCY, AND A FURTHER LAYER IS THEN CAST ON TOP OF THE PRECEDING SELF-SUPPORTING LAYER, AN IMPROVEMENT COMPRISES, IN THE INTERVAL BETWEEN THE CASTING OF THE TWO LAYERS, THE STEP OF APPLYING TO THE UPPER SURFACE OF THE FIRST LAYER A FILM OR COVERING WHICH AT LEAST DURING THE CASTING PROCESS REMAINS ESSENTIALLY TIGHT OR RETARDANT TO THE PENETRATION OF LIQIDS. THIS ELIMINATES DIFFERENCES OF TEXTURE AND UNEVEN FORMATION OF BUBBLES AT THE CASTING SEAM.

United States Patent O 3,808,299 METHOD OF CASTING AERATED CONCRETE PRODUCTS Sven-Olof Svensson, Hallsberg, Sweden, assignor to Intong Aktiebolag, Hallabrottet, Sweden No Drawing. Filed Dec. 10, 1971, Ser. No. 206,856 Claims priority, application Sweden, Dec. 10, 1970, 16,709/ 70 Int. Cl. B29h 7/20 US. Cl. 264-42 9 Claims ABSTRACT OF THE DISCLOSURE In a method of casting aerated concrete products wherein a first aerated concrete mass layer is cast and permitted to set or harden into a self-supporting consistency, and a further layer is then cast on top of the preceding self-supporting layer, an improvement comprises, in the interval between the casting of the two layers, the step of applying to the upper surface of the first layer a film or covering which at least during the casting process remains essentially tight or retardent to the penetration of liquids. This eliminates differences of texture and uneven formation of bubbles at the casting seam.

FIELD OF THE INVENTION The present invention relates to an improved method of casting aerated concrete products, and particularly to an improved method of casting aerated concrete in two or more layers.

BACKGROUND OF THE INVENTION Aerated concrete products are usually obtained by pouring an aerated concrete mass, chiefly consisting of a siliceous raw material, a limy binding agent, a porosityforming agent and water, into casting moulds of suitable dimensions. The siliceous material may be, for instance, a fine-ground sand, shale ash, fly ash or other materials with a high content of SiO The binding agent may be slaked and/ or unslaked lime or portland cement. Also some combinations of lime and cement may be advantageous. The porosity-forming agent preferably consists of aluminum powder, which, its surrounding milieu having been made alkaline by the binding agent, develops hydrogen gas in connection with the casting procedure, thus bringing-about the characteristic bubbles or porosity of the aerated concrete. The casting mould may advantageously measure 600 x 120 x 60 cm., the area of the mould bottom being 600 x 120 cm. and the form side walls of the mould 60 cm. high. When the mass has been cast into a mould it is made porous by this hydrogen gas development, the degree of the porosity depending on the quantity of the aluminum powder. After that, the mass is left for some time to set or harden to a semi-plastic, self-supporting consistency. The mass thus having set to form a self-supporting body, the mould or some parts of the same are taken away from it and the exposed body is divided up and fashioned into building elements of the desired shape, which is suitably carried out by means of thin wires making longitudinal, vertical cuts in it. Finally the divided and fashioned body is given a curing treatment, preferably by steamcuring in an autoclave.

In some cases it is possible to cast all the aerated concrete mass needed for filling up the mould, in one step only. If for instance a mass is to be cast, the final volume of which when made porous will be twice the size of the original one, the mass is filled-in to half the height of the mould, i.e., 30 cm., the remaining 30 cm. being covered by the rising of the mass. In other cases it will be suitable or necessary to divide the casting process Too into two or more steps, the mass of the first stepbeing given time to set to self-supporting consistency before casting the mass of the next step. Such dividing of the casting process means, among other advantages, that a more even porosity is obtained than when casting in one step only, and also that the mass effectively encloses and embeds the reinforcements, if any.

-It has, however, turned out that disadvantages may at times appear when casting in several steps. One is that those parts of the aerated concrete which are situated near to the seam or boundary zone between two layers of mass, may develop a texture differing from the other parts and in certain cases diverse types of cavities may be formed, likely to reduce the strength. Another possible fault is that the coherence in the seam between the layers will not be satisfactory after the steam-curing.

SUMMARY OF THE INVENTION The present invention relates to a casting method eliminating the above disadvantages of the casting in two or more steps. Characteristic of the method invented is that in the interval between the casting of two layers of mass there is applied to the upper surface of the first layer a film or covering which at least during the casting process remains essentially tight or retarding to the penetration of liquids.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Tests carried out have shown that the reason for difference of texture and uneven formation of bubbles at the casting seam is that the water at hand in the lower layer of the mass becomes more or less bound during the setting of the mass, and water from the upper layer of the mass is sucked down by the lower layer being drier, causing disturbances to the porosity formation process. By means of the process according to the invention this phenomenon can be mastered as the film applied to the surface of the first layer will prevent water from being transported from the upper layer to the lower layer during the critical time until the upper layer has set. It has been ascertained that absolute tightness of the film to the penetration of liquids is not necessarily required, but only a more or less strong retardation of water transport between the layers. The essential thing is that the marked water transport which occurred earlier in multi-stage castings is prevented.

According to a particularly advantageous embodiment of the invention the film applied to the layer of mass in question is strongly adhesive to the aerated concrete even after the steam-curing. In that way the cohesion between the different layers is improved.

The film to be applied may be produced in different ways and from different materials. It may, according to the invention, consist of or comprise plastic or rubber. One possibility, for instance, is to use styrene-butadiene latex, which suitably may be kept dispersed in water at the applying occasions. Further it is possible to use polyvinyl alcohol as well as various sorts of acrylic plastic, such as polybutyl metacrylate, polymethyl acrylate or polyethyl acrylate. The thickness of films made from such plastics has been determined experimentally as corresponding to a quantity of plastic of 5-100, preferably 30-50, g./m. of solids at hand in the material.

Instead of plastics or rubber materials also gluey materials may be used, such as ethyl-hydroxyethyl cellulose.

To such film-forming materials as mentioned above it is possible, if desired, to mix various types of inorganic additives. Suitable ones are, for instance, portland cement or fine-ground materials containing SiO such as sand and sandstone. Furthermore, there may be mixed-in fineground lime or aerated concrete mass which has not yet been made porous, as this mass chiefly consists of a mixture of a siliceous raw material with a limy binding agent. It is possible also to make films or covering layers exclusively from one or more inorganic materials, i.e., not containing any plastic or rubber.

It is most advantageous according to the invention to apply the film perpendicularly to the plane of a building element to be taken from the cast block of mass. This may be done by applying the film to the upper surface of the first layer which surface is essentially horizontal and then dividing up the cast block by vertical cuts. It may be desired, for instance, to produce 6 elements 600 cm. long and 60 cm. wide from a mould of the dimensions 600 x 120 x 60 cm. It is possible then to cast a first layer of mass with a height, after completed porosity formation, of 30 cm. and to apply a film onto the horizontal upper surface of the same. on top of the first layer, then, a second layer of equal height is cast, i.e., the mould is filled up to its upper edge after completed porosity formation. After that, the block thus formed is, by five vertical cuts, divided into six elements of the dimensions 600 x 60 x 20 cm., the plane of the seam or the film thus becoming situated crosswise to the big surface or plane of the elements.

The applying of the fihn can in practice be carried out by spraying the film-forming material with an optional spraying device.

In the following, two examples are given of practical application of the invention.

EXAMPLE I In a casting mould measuring 600 x 120 x 60 cm. 1.1 m. of such a type of aerated concrete mass is poured as is redoubled by the porosity formation. Almost immediately after being poured the mass will have risen by the formation of pores into a layer 30 cm. high. This layer is left to set for 40 minutes which is enough for the layer to obtain suflicient strength for the casting of the next layer. At some point of time during the setting period there is applied to the top surface of the layer of mass an all-covering film consisting of styrene butadiene latex, quantity: 40 g./m. The setting completed, the latex has formed a liquid-retarding film and a further 1.1 m? of aerated concrete mass is poured on top of it and is then left to form pores and to set. At a special dividing-up station the block thus formed is divided into a suitable number of elements which, finally, are steam-cured in an autoclave with steam of the temperature 180 C. and the corresponding over-pressure for 12 hours.

EXAMPLE 11 Instead of the styrene butadiene latex of Example I a film-forming material is-made from a mixture of 4 parts by weight of a 50% polymethyl metacrylate dispersion, 2 parts by weight of portland cement and 6 parts by weight of water. A quantity of 100 g./m. of this mixture is applied to a first layer of mass.

Aerated concrete elements produced according to' the above examples show an even porosity on both sides of the seam and, besides, the cohesion between the layer cast-together is extraordinarily good.

Beside the advantages earlier mentioned the invention offers considerable advantages as to the casting technique. Heretofore it has involved great difiiculties exactly to choose the correct point of time for casting the second or subsequent layer. If it was applied to early, there was the risk of the first layer collapsing, if too late, too much water would pass between the layers causing faults in the aerated concrete nearest to the casting seam. For some raw material it has earlier been necessary to cast the second layer already 20-25 minutes after the first one to avoid the occurrence of unacceptable deficiencies around the casting seam. This has of course meant an imminent risk of the first layer crumpling down as that period is too short for the complete setting of the aerated concrete. These disadvantages are prevented completely by the invented method by which it is possible to let the first layer set to satisfactory hardness or strength without the occurrence of any water transport worth mentioning.

It is self-evident that the invention may be modified in many ways within the scope of the following claims.

What is claimed is:

1. In a method of multilayer casting aerated concrete products comprising the steps of first casting one aerated concrete mass layer which is then brought to set or harden into a self-supporting consistency, then in at least one second step casting a further layer of aerated concrete mass on top of the preceding self-supporting layer and permitting said f urther layer to set or harden into a self-supporting consistency, and steam-curing the multilayer body, the improvement comprising improving the porous structure in the joint area between the two layers while promoting adhesion between the two layers by, in the interval between the casting of the two layers of mass, the step of applying to the upper surface of the first layer a solution or liquid dispersion of a film-forming polymeric material which, at least during the casting process, at least retards migration of water from said further layer to said first layer, and which promotes adhesion between the two layers after steam-curing.

2. A method according to claim 1 wherein the filmforming material comprises styrene butadiene latex.

3. A method according to claim 1 wherein the filmforming material comprises polyvinyl alcohol.

4. A method according to claim 1, wherein the filmforming material comprises acrylic plastic.

5. A method according to claim 1 wherein the filmforming material comprises ethyl-hydroxyethyl cellulose.

6. A method according to claim 1 wherein the filmforming material is applied to the surface in a quantity corresponding to 5-100 g./m. calculated as weight of the solids at hand in the material.

7. A method according to claim 6 wherein said quantity corresponds to 30-50 g./m.

8. A method according to claim 1 wherein said filmforming material includes inorganic material additives selected from the group consisting of portland cement, finely-ground materials containing silicon dioxide, and finely-ground lime.

9. A method according to claim 1 including the step of cutting the multi-layer body along planes normal to the film layer to form aerated concrete building elements containing said first and further layers adhered together at said joint.

References Cited UNITED STATES PATENTS 3,000,276 9/1961 Foulger 94-10 2,531,990 11/ 1950 Rappoli 264-Dig. 057 2,531,576 11/1950 McClellan et al. 264-256 3,595,947 7/ 1971 Wielinga 264--42 MELVYN I. MARQUIS, Primary Examiner US. Cl. X.R.

264-157, 256, 271, 333, 338, Dig. 57 

